The pore size distribution and the surface of intrafibers have been measured by many workers using various methods. In this report the mercury-intrusion method adopted by Quynn and Burleigh was modified to extend the pressure range from 50 to 3000 kg/cm2 and was applied to the measurement of pore size of viscose and bemberg rayon filaments. Two kinds of mercury porosimeter were used: the one was for measuring up to 500kg/cm2 and the other up to 3000kg/cm2.In the range of pore diameter from 0.3 to 0.025 μ corresponding to the pressure range from 50 to 600kg/cm2, the total pore volume of viscose rayon was 0.0031cc/g whereas that of bemberg rayon was larger, 0.0037cc/g. On the other hand, in the range of pore diameter from 0.025 to 0.005 μ corresponding to the pressure range from 600 to 3000kg/cm2, the total pore volume of viscose rayon was 0.0129 cc/g whereas that of bemberg rayon was smaller, 0.0107cc/g.The measured values up to about 50kg/cm2 were dispersed. This fact seems to be a result of absorption of mercury on the surface of fibers and of intrusion of mercury in the interfiber spaces. In the pressure range at which mercury is absorbed, the influence of the bottle-neck pores or isolated pores should be considered, but such influence may be small because the repeated measurements of a sample were fairly reproducible.Further, at the higher pressure the influence of compression of fibers per se or the deformationn of pores should be considered, and these controversial points are discussed.